@marcus : some people think Picasa is not a priority for Google:
"Or take the case of Googleâs Picasa, a Windows program that finds, organizes, and shares photos. Since that product mirrors much of the functionality of Appleâs iPhoto, there would seem to be little incentive for Google to create a Mac version. Yet, rather than ignore the platform, Google built a photo uploader that works with iPhotoâ”another 20-percent projectâ”to allow Mac users to connect iPhoto to the Picasa Web Album feature for sharing photos online."
From: Macworld - Googleâs Mac efforts begin to bear fruit.